Structural Changes during Mechanical Mixing in Carbon Black-Natural Rubber Systems Studied by Pulsed NMR

Abstract

ABSTRACT:Proton spin-spin relaxation time (T 2 ) has been measured by the pulsed NMR technique for composites of natural rubber and carbon black prepared by Brabender mixing. The amounts and mobilities of tightly bound rubber and loosely bound rubber, which are determined by the free induction decay analysis, change with mixing time. At the first stage of mixing, the amounts of both components increase rapidly and the mobility of the tightly bound rubber decreases. The ratio of the tightly bound rubber to the loosely bound one decreases rapidly with time, reaching a limiting value at the black incorporation time. At the second stage, the amounts continue to increase at a slower rate but the mobilities become independent of the mixing time. At the third stage, no appreciable change occurs in the amount of each component. Thus, the tightly bound rubber appears to be formed very quickly when the rubber segments get in contact with a new active surface of carbon black, followed by the rapid growth of the loosely bound rubber component around the surface of the preformed tightly bound rubber phase.KEY WORDS Natural Rubber I Carbon Black I Mechanical Mixing I Pulsed NMR I Spin-Spin Relaxation Time I Segmental Mobility I Tightly Bound Rubber I Loosely Bound Rubber I Incorporation of surface active carbon black into various kinds of rubber improves the mechanical properties (tensile modulus and strength) as a result of the interactions between carbon black and rubber segments. 1 -3 When the carbon black is incorporated into rubber by mechanical mixing, a significant amount of rubber becomes insoluble due to the strong interactions induced during mixing. 4 This insoluble component is usually called "bound rubber." Pulsed NMR studies 5 -7 have revealed that the bound rubber is composed of two phases having different mobility; i.e., tightly and loosely bound rubber phases. The tightly bound rubber phase is formed around the surface of carbon black particles. The thickness of this phase was estimated to be about 4.5 nm for ISAF grade carbon black, although it changed depending on the nature of carbon black used. 8 Thus, the pulsed NMR studies provided very important information on the structure of the bound rubber phases produced by mechanical mixing. However, no NMR study has been reported on the structural changes during mechanical mixing. In this work, proton spin-spin relaxation time (T2) of the bound rubber was measured as a function of the mixing time by the pulsed NMR technique. On the basis of these results, the mechanism of the bound rubber formation during mixing was discussed in terms of the changes in the segmental mobility and the fractions of the tightly bound rubber (short T 2 ) and loosely bound rubber (long T2) components. The changes were followed by the mixing torque which is one of the most convenient and sensitive measures of the overall structural change during mixing. 149

Chain Scissions of Rubber Molecules by a Tensile Force in Filled Rubber Systems

“…It is well known that the filled rubber vulcanizates have multi-component structure depending on the chain mobility of rubber molecules [11][12][13] . A pulsed NMR is a powerful technique to obtain information on the chain mobility in each component and the component fraction of the multi-component systems.…”

“…A pulsed NMR is a powerful technique to obtain information on the chain mobility in each component and the component fraction of the multi-component systems. For example, carbon black filled SBR vulcanizates exhibited three T 2 Јs, the long T 2 (T 2L ), intermediate T 2 (T 2M ) and short T 2 (T 2S ), indicating the existence of loosely, intermediately and tightly restricted rubber molecules [11][12][13] . Thus, the NMR results are expected to tell us which component will be involved in the chain scissions by a tensile force.…”

“…Further, it was found that the degree of chain scission was closely related to the strength of interfacial bonding which was controlled by the coupling agent. It is well known that the filled rubber systems have multi-component structure depending on the chain mobility of rubber molecules [11][12][13] . Information on the chain mobility of each component and the component fraction can be obtained by the pulsed NMR method [11][12][13] .…”

Effect of rubber polarity on selective wetting of carbon nanotubes in ternary blends

“…That leads to a significant and uncontrolled change in the surface energy of the filler and thus its wettability [6]. Different methods [7][8][9][10][11][12][13][14][15][16][17] were developed and used for characterization of filler wetting and polymer-filler interaction in nanocomposites containing CNTs and single polymer. Scanning electron microscopy (SEM) of cryo-fractured surfaces of the composites was used for visualization of the polymer layer bonded to the filler [7,8].…”

“…The shift of the D and G bands in the Raman spectra of the CNT composites was discussed as a result of the CNT-matrix wetting [9,10] and cation-# bonds between ionic liquid and CNTs [11]. Analysis of rubber-filler gel after extraction experiments (bound rubber measurement) by means of nuclear magnetic resonance (NMR), pyrolysis gas chromatography (PGC) and Fourier transform infrared spectroscopy (FTIR) as well as thermogravimetric analysis (TGA) was carried out for qualitative and quantitative characterization of the physical background of the filler-polymer interaction in carbon black or silica filled rubber compounds [12][13][14][15][16][17]. If the rubber matrix consists of two or more rubber components the wetting of filler aggregates becomes much inhomogeneous because of the different affinity of filler to rubber components.…”